I have one office’s pc that use some motherboard with MCP61 sound card. I knew it when I do lspci in the terminal. I m using Ubuntu 8.10. It seems the alsa driver can’t handle it. And I discovered in bugs.launchpad.org it was recorded as the bug. The sound won’t come out, and the computer can’t shutdown normally. The screen hung when shutting down alsa. I discovered this when the screen freeze in shutdown process, with pressing ctrl+alt+F1 (or F2 or F3) to switch to another tty console.
So there is a shortcut that I used to force alsa to shutdown. Meanwhile it is recorded as bug. Simply download the file here and replace /etc/init.d/alsa-utils with this file. Try to shutdown again, it should work.